The invention relates to novel manganese dioxide electrodes comprising modified, electrochemically active manganese dioxide, to a method for fabricating these novel manganese dioxide electrodes and to their use in rechargeable cells.
Typical components of an alkaline primary cell are a cathode consisting of manganese dioxide, an anode preferably made of zinc, an alkaline electrolyte and an electrolyte-permeable separator material. The manganese dioxide which is customarily used for fabricating the cathode, being an electrolyte manganese black having a xcex3-structure, exhibits very high activity. The discharge mechanism of manganese dioxides has been studied very thoroughly in the past. An overview of such studies is provided, for example, by A. Kozowa in: Batteries, Vol. 1, Manganese Dioxide, Ed.: K. V: Kordesch, Chapter 3, Marcel Dekker 1974. The use of manganese black in alkaline zinc-MnO2 cells from a commercial point of view has been set forth by R. F. Scarr and J. C. Hunter in: Handbook of Batteries, Ed. D. Linden, Chapter 10, McGraw-Hill 1995.
To increase the electrical conductivity, such manganese dioxide electrodes are customarily admixed with particles of coal, carbon black or graphite. Organic or inorganic additives are used as binders.
The zinc electrode as a rule consists of zinc powder having a large surface area and of a gelling agent, e.g. carboxymethylcellulose, as the stabilizer. Also known, however, are cold-pressed or hot-pressed elctrodes, with or without a binder, or sintered zinc powder electrodes. In addition to amalgamated zinc anodes, mercury-free zinc anodes are increasingly being used.
The alkaline electrolyte commonly consists of an aqueous potassium hydroxide solution. Alternative options, however, are solutions of other hydroxides such as sodium hydroxide solutions or lithium hydroxide solutions and mixtures thereof.
The separator material present between the electrodes has the purpose of electronically insulating the two electrodes.
On economic and ecological grounds, attempts have been made for some time to fabricate a rechargeable alkali-manganese cell.
The greatest problem in this context has so far been posed by the manganese dioxide electrode, since it does not have an adequate discharge/charging cycle stability. During discharge, protons are inserted into the MnO2 lattice, as a result of which xcex3-MnO2 is reduced to the isostructural xcex1-MnOOH. This takes place in a homogeneous solid-phase reaction and entails expansion of the manganese black structure. If a further discharge occurs, the stability limit is exceeded, i.e. the expanded structure collapses and phases are formed which have a different crystal structure (e.g. Mn(OH)2, Mn3O4). This is an irreversible collapse of the MnO2 crystal lattice. After this, recharging is no longer possible. Changes in the lattice structure of the xcex3-MnO2 commence at a degree of intercalation of about 0.8 protons per manganese atom. In the case of known rechargeable MnO2xe2x80x94Zn cells this is taken into account by the discharge being limited either by the final discharge voltage of about 0.9 V or by the cyclable amount of zinc (K. Kordesch; K. Tomantschger and J. Daniel-Ivan in: Handbook of Batteries, Ed.: D. Linden, Chapter 34, McGraw-Hill 1995).
DE 33 37 568 A1 describes the preparation of xcex3-MnO2 by direct-current electrolysis from manganese(II) salt solutions, which had been admixed with titanyl ions. Using the MnO2 thus produced, it is possible, according to the description, for the number of discharge/charging cycles for a discharge level of 33% in alkaline aqueous electrolytes to be more than 100 cycles, more than doubled that of the reference manganese black (International Common Sample (ICS) No. 2) at 42 cycles. The publication points out that it is not possible simply by admixing the manganese black with titanium dioxide powder, to effect an improvement in the cycling capability.
EP 0 146 201 A1 describes the addition of bismuth ions and lead ions to the manganese black, as an activator. This publication claims the intensive mechanical admixing of ICS manganese black and of Na birnessite (Na, MnxO4) with Bi2O3 (or Bi2S3) and PbO. Given a discharge level of 50% (based on the theoretical 2 exe2x88x92 capacity) the material obtained exhibits good cyclability. The electrodes fabricated using the material described do have a very high excess of graphite, and consequently their use in practice is likely to be problematic. The publications EP 0 136 172 A2 and EP 0 136 173 A2 describe the use of corresponding modified manganese dioxides in MnO2xe2x80x94Zn cells.
PCT/WO92/14273 A1 describes a method for preparing manganese black which is rechargeable in aqueous solution, said method involving the impregnation or mixing of conventional xcex3- or xcex2-manganese black with an aqueous solution of bismuth ions, lead ions or copper ions.
PCT/WO93/12551A1 discloses barium additives, especially barium oxide, barium hydroxide and barium sulphate as an additive to cathode pastes for alkaline zinc-manganese dioxide cells. As a result of the addition of these barium compounds, higher cumulative capacities are achieved compared with standard cells. The method of operation is described by stating that the barium ions located in the vicinity of active cathode material impede access of the zinc ions to the manganese dioxide and thus prevent or retard the formation of electrochemically entirely inactive hetaerolite (ZnOxc3x97Mn2O3). Also discussed is a positive effect on the porosity of the manganese black electrode. Corresponding barium additives are also described in U.S. Pat. No. 5,424,145 A. This publication likewise discloses the addition of oxides, spinels and perovskites, especially compounds of nickel, cobalt, aluminium, zinc, iron, manganese, chromium, vanadium, titanium and silver. The purpose of these additives is to protect the manganese dioxide against overcharging and to prevent further oxidation to higher-valence manganese compounds such as manganates.
It is therefore an object of the invention to provide cyclically stable MnO2 electrodes which are suitable for use in rechargeable alkaline cells. Another object of the invention is to provide, for the fabrication of manganese dioxide electrodes, inexpensive, suitably modified manganese dioxide which, given an effective discharge level has adequate cycle stability, ideally without any change in the lattice structure, exhibits improved reversibility characteristics compared with conventional material while discharge times are extended and cell voltages during the discharge are increased, this applying both to high and to low discharge rates. A further object of the invention was to provide an inexpensive and simple method for fabricating these modified manganese dioxide electrodes.
This object is achieved by manganese dioxide electrodes fabricated from a conventional manganese dioxide whose characteristics have been modified beforehand by the addition of coated inorganic particles.
These coated inorganic particles may be of a type where the support particles consist of a material selected from the group consisting of mica, SiO2, Al2O3, ZrO2 and ZnO. Single or multiple coatings of these particles may be composed of dielectric substances and in particular of ferroelectric, piezoelectric or pyroelectric substances. Such coatings may consist of titanates, stannates, tungstates, niobates or zirconates; also possible, however, are silicate coatings, depending on the type of the base particles selected. Particles with coatings from mixtures of these substances are likewise suitable. Suitable inorganic particles may also have coatings consisting of metal oxides from the group consisting of Fe2O3, NiO, CoO, ZrO2, SnO2, TiO2, Sb2O3, PbO, Pb3O4, Bi2O3, or mixtures thereof. The individual coatings which, per se, consist of one substance may be doped with foreign ions, an example being SnO2 coatings doped with foreign ions. The manganese dioxide used as the base material may be present in a structure which contains water of crystallization.
The abovementioned object is achieved, in particular, by manganese dioxide electrodes comprising coated inorganic particles in an amount of from 0.01 to 20% by weight, based on the amount of manganese dioxide comprised in the electrode.
The invention therefore relates to manganese dioxide electrodes modified by the addition of inorganically coated particles and to the use of these electrodes in rechargeable cells, in particular rechargeable alkaline batteries in which, by preference, zinc electrodes are used as counter-electrodes.
The invention also relates to a method for fabricating the novel manganese dioxide electrodes.
The manganese dioxide electrodes are fabricated by
a) the manganese dioxide powder being homogenized with an inorganic powder consisting of singly or multiply coated inorganic particles,
b) the mixture being, if required, mixed with an organic or inorganic binder and a conductive addition, preferably carbon black or graphite, and
c) the product obtained being processed to give an electrode.
Manganese dioxide electrodes according to the invention not only can be used for the fabrication of galvanic elements, electrochemical cells, of primary batteries, e.g. button cell batteries, but also exhibit particularly good characteristics in rechargeable cells, in particular rechargeable alkaline batteries.
Surprisingly, trials have shown that mixing of the manganese dioxide customarily used as a cathode material with commercially available inorganic coated particles, so-called perlescent pigments gives a starting material for fabricating manganese dioxide electrodes, from which cathodes or positive electrodes having considerably improved characteristics can be fabricated. These pigments are inorganic particles which can be coated with a wide variety of substances.
The trials carried out have shown that an addition of inorganic coated particles affords cathodes or positive electrodes having extended discharge times, if the manganese dioxide is admixed with from 0.01 to 20% by weight, based on the amount of manganese dioxide, of such inorganic particles in the form of coated mica particles, SiO2 particles, Al2O3 particles, ZrO2 particles or coated ZnO particles. The amount added in each case depends on the intended use of the manganese dioxide electrodes after fabrication. Whilst even the addition of a small amount of about 0.01% by weight of the abovementioned particles has a significant effect on the discharge times of commercially available batteries, additions of up to 20% by weight to cathode materials of button cell batteries can be worthwhile.
Modifying the cathode material achieves an increase in the primary capacity of the electrochemical cell by from 10 to 30% compared with commercially available zinc/manganese oxide batteries, whose cathodes are not modified. Moreover, modifying the electrode material makes it possible to achieve significantly improved rechargeability of electrochemical cells. An increase in the primary capacity by 30% or an increase in the rechargeability by 200% can be achieved, in particular, by the addition of 20% by weight of inorganic coated particles to the manganese dioxide employed. Accordingly it may be advisable to vary the amount added depending on the use to which the electrodes are put.
Found to be particularly suitable for modifying the manganese dioxide employed in the fabrication of electrodes were commercially available coated inorganic particles comprising mica as the support material. Such materials are:
mica coated with titanium dioxide in the anatase or rutile modification
mica coated with SiO2 and/or SnO2 and/or TiO2 
mica coated with alkaline earth metal titanates (Mg, Ca, Sr, Ba titanates), alkali metal titanates and/or lead titanate and/or manganese titanate
mica coated with stannates, tungstates, niobates or zirconates
mica coated with metal oxides (Fe2O3, NiO, CoO, ZrO2, SnO2, Sb2O3, PbO, Pb3O4 or Bi2O3)
mica coated with ZrO2 
mica coated with mixtures of these oxides and titanates.
Also suitable, however, for modifying are inorganic particles coated in the same way, in which particles of SiO2, Al2O3, ZrO2 serve as the support material. Good effects are achieved with the aid of particulate materials whose support materials are polarizable even per se, although this is not a prerequisite since improved capacities are measured even with materials whose support particles do not have these characteristics. It has been found to be particularly advantageous, however, for the coatings to consist of dielectric substances, in particular ferroelectric, piezoelectric or pyroelectric substances such as titanates, stannates, zirconates, tungstates, niobates or others.
Surprisingly, experiments have shown that the use, according to the invention, of particles with titanium dioxide coatings, in contrast to U.S. Pat. No. 5,342,712, results in a considerable increase in capacity of the experimental cell, irrespective of whether the coating has an anatase or a rutile structure. These experimental results also contrast with Application DE 33 37 568 A1, according to which a manganese dioxide modified mechanically with TiO2 powder does not have any positive effect in terms of increased cyclability. It was therefore found, surprisingly, that as a result of the addition of inorganic particles which consist of a support material as mentioned above and a titanium dioxide layer applied on top of this, a distinct increase in the cyclability of the modified manganese dioxide is achieved, compared with conventional materials.
It was also found that the advantageous result is achieved without highly pure starting substances being used for modifying. Uniformly good results are achieved if the surfaces of the particles used for modifying the electrode material are coated with metal oxides from the group consisting of Fe2O3, NiO, CoO, ZrO2, SnO2, TiO2, Sb2O3, PbO, Pb3O4, Bi2O3, WO3, NbO or with mixtures of these metal oxides. Surprisingly good increases in the primary capacity and the cyclability are achieved by the addition of particles whose surface coatings are doped with foreign ions, such as SnO2 coatings doped with antimony.
According to the present invention, simple mixing of a pulverulent additive with an inexpensive, commercially available pulverulent manganese dioxide affords a starting material for the fabrication of electrodes having improved cyclability.
To prepare the electrode material proper, the manganese dioxide powder is mixed with the desired quantity of particulate powder and is homogenized in a manner known to those skilled in the art. Homogenization can be effected by grinding in ball mills or attritors. Grinding using ball mills over a period of about eight hours or more proved effective in the trials carried out. The product thus homogenized can then be mixed with further additives, e.g. with organic or inorganic binders and conductivity additives. Organic binders which can be added for this purpose include PTFE, latex and other binders known to those skilled in the art. Of possible use as an inorganic binder is Portland cement. Particularly suitable is PTFE. Suitable conductivity additives include carbon black, graphite, steel wool and other conductive fibres. Particularly good results were achieved by the addition of carbon black and graphite in an amount of 4-10, in particular of about 5% by weight, based on the total amount.
Then the powder mixed with all the additives is processed, in a manner known per se, to produce electrodes. This can be effected by pressure-moulding at very high pressure between wire cloths consisting of an inert material such as nickel. Where appropriate, this may be followed by a treatment at elevated temperature, a so-called tempering procedure.
Electrodes thus fabricated can be used, in a known manner, for fabricating rechargeable cells in the presence of an alkaline electrolyte, in which the counter-electrode used can be a zinc electrode. Alternative designs of corresponding galvanic cells are also possible, however. For example, various additives such as gelling agents, silica gel or others can be employed to increase the viscosity of the essentially aqueous electrolyte. A suitable polymer fabric or bonded polymer batt can be fitted as a separator material between the electrodes and, should this be necessary, a spacer should be inserted. The polymer batt used can be in the form of materials consisting of poly(vinyl acetate), polypropylene or other inert polymers. Spacers such as those known from commercially available batteries may be of corrugated shape and consist, for example, of PVC.
For experimental purposes, the manganese dioxide mixtures according to the invention were made into electrodes by means of the addition, after grinding, of both a conductivity additive and a binder. The mixture thus obtained was pressure-moulded between two nickel wire meshes to produce cathodes.